WASHINGTON – Barack Obama stood at the threshold of the White House on Monday, summoning fellow Americans to join him in service as tens of thousands flocked to the nation’s capital to celebrate his inauguration as the first black president.

“Tomorrow we will come together as one people on the same Mall where Dr. King’s dream echoes still,” said the president-elect, invoking the memory of the Rev. Martin Luther King Jr. on the national holiday in his honor.

The 47-year-old former senator takes office at noon Tuesday at a time of economic crisis at home and two wars overseas. But the challenges of the moment receded into the background for many who came for a chance to witness history.

“I didn’t think I would see a black president in my generation. I just had to be here,” said Donald Butler, 20, a University of Washington student.

“I’m just really happy that I’m living to see this wonderful event,” said 70-year-old Betty Bryant, who rode a chartered bus from Augusta, Ga. Standing in front of the icy Reflecting Pool facing the Capitol, she made plans to rise at 3 a.m. Tuesday to take her place on the Mall for the swearing-in.

Obama’s own schedule showed he had one eye on the bipartisan support he’ll need after taking office. The night before becoming the nation’s 44th president he was stopping by dinners saluting his vice president, Joe Biden, and two Republican stalwarts: former Secretary of State Colin Powell and Sen. John McCain of Arizona, the man Obama defeated to win the White House.

Uniformed military personnel patrolled Washington street corners, the advance guard of a massive security presence planned for the oath-taking, inaugural speech, parade and other festivities. Officers checked out some suspicious packages and vehicles, but everything was cleared, said FBI spokesman Richard Kolko. “The city and the people seem to be in a good mood and good spirits,” he said. “Security is going well, that’s what all the planning is for.”

George W. Bush was in the White House for a final full day as president after two terms marked by the terror attacks of Sept. 11, 2001, and the economic collapse of 2008. He placed calls to world leaders, including Russian Prime Minister Vladimir Putin, and commuted the prison sentences of two former Border Patrol Guards.

On Tuesday morning, he and first lady Laura Bush will greet Obama and his wife, Michelle, and see them in for a brief visit. By the time Obama returns mid-afternoon, he will be the nation’s 44th president, Bush will be en route to a Texas retirement and the moving vans will have departed with one family’s belongings and arrived with the other’s.

Associated Press Writers Nedra Pickler, Jesse Holland, Charles Babington, Eileen Sullivan and Larry Margasak contributed to this report